Neutral-aged hydrophobic organosilicate-modified silica gels

ABSTRACT

The present invention is neutral-aged hydrophobic organosilicate-modified silica gels and a method for their preparation. The method comprises three steps, where in the first step an organosilicate-modified silica hydrosol is contacted with a strong mineral acid at a pH less than about 1 to form an organosilicate-modified silica hydrogel. In the second step the organosilicate-modified silica hydrogel is aged at a pH within a range of about pH 3.5 to pH 8. In the third step the neutral-aged organosilicate-modified silica hydrogel is contacted with an organosilicon compound in the presence of a catalytic amount of a strong acid to effect hydrophobing of the organosilicate-modified silica hydrogel to form a hydrophobic organosilicate-modified silica hydrogel having a surface area within a range of about 100 m 2  /g to 750 m 2  /g in the dry state. In a preferred process the hydrophobic organosilicate-modified silica hydrogel is contacted with a sufficient quantity of a water-immiscible organic solvent to convert the hydrophobic organosilicate-modified silica hydrogel into a hydrophobic organosilicate-modified silica organogel.

BACKGROUND OF INVENTION

The present invention is neutral-aged hydrophobicorganosilicate-modified silica gels and a method for their preparation.The method comprises three steps, where in the first step anorganosilicate-modified silica hydrosol is contacted with a strongmineral acid at a pH less than about 1 to form anorganosilicate-modified silica hydrogel. In the second step theorganosilicate-modified silica hydrogel is aged at a pH within a rangeof about pH 3.5 to pH 8. In the third step the neutral-agedorganosilicate-modified silica hydrogel is contacted with anorganosilicon compound in the presence of a catalytic amount of a strongacid to effect hydrophobing of the organosilicate-modified silicahydrogel to form a hydrophobic organosilicate-modified silica hydrogelhaving a surface area within a range of about 100 m² /g to 750 m² /g inthe dry state. In a preferred process the hydrophobicorganosilicate-modified silica hydrogel is contacted with a sufficientquantity of a water-immiscible organic solvent to convert thehydrophobic organosilicate-modified silica hydrogel into a hydrophobicorganosilicate-modified silica organogel. The organic solvent can thenbe removed from the organogel to form a hydrophobicorganosilicate-modified silica gel having a surface area within a rangeof about 100 m² /g to 750 m² /g in the dry state. A water solublecompound of cerium or iron may be added in the third step to improve theheat stability of the hydrophobic organosilicate-modified silica gel.

Although hydrophobic organosilicate-modified silica gels prepared by thepresent method are useful in many applications such as thermalinsulation, reinforcing and extending filler in natural rubbers, and asfiller in floatation devices, they are particularly useful asreinforcing fillers in silicone rubber compositions. It is well knownthat silicone rubbers formed from the vulcanization ofpolydiorganosiloxane fluids or gums alone generally have low elongationand tensile strength values. One means for improving the physicalproperties of such silicone rubbers involves the incorporation of areinforcing silica filler into the fluid or gum prior to curing.However, silica reinforcing fillers have a tendency to interact with thepolydiorganosiloxane fluid or gum causing a phenomenon typicallyreferred to as "crepe hardening." A great deal of effort has been madein the past to treat the surface of reinforcing silica fillers withorganosilanes or organosiloxanes to make the surface of the silicahydrophobic. This surface treatment reduces or diminishes the tendencyof the compositions to crepe harden and improves the physical propertiesof the cured silicone rubber.

Brown, U.S. Pat. No. 3,024,126, teaches a method for making a pre-formedreinforcing silica filler hydrophobic by treating it in an organicsolvent with an organosilicon compound, such as an organosilane orlow-molecular weight organosiloxane containing 0.1 to 2 total hydroxyland/or alkoxy radicals per silicon atom, and a small amount of amine,quaternary ammonium, or organometallic compound.

Lewis, U.S. Pat. No. 3,979,546, teaches a method for making the surfaceof reinforcing silica fillers hydrophobic through the use ofalpha-alkoxy-omega-siloxanols with alcohols under mild conditions. Thefillers taught are pre-formed solids.

Tyler, U.S. Pat. No. 3,015,645, teaches the making of hydrophobic silicapowders by reacting an organosilicon compound such asdimethyldichlorosilane or trimethylmethoxysilane with an silicaorganogel in the presence of an acidic catalyst and then removing thevolatile materials. The method requires the preparation of a silicahydrogel which is converted to a silica organogel by replacing the waterin the silica hydrogel with an organic solvent.

Lentz, U.S. Pat. No. 3,122,520, teaches a procedure where an acidicsilica hydrosol is first heated to develop a reinforcing silicastructure and then mixed with an organosilicon compound, an acidcatalyst, and a water-immiscible organic solvent to produce ahydrophobic silica filler. The organosilicon compounds taught by Lentzare limited to those compounds in which the organic radicals bonded tosilicon atoms have less than 6 carbon atoms, organosilicon compoundshaving no organofunctional substituents bonded to silicon atoms, and toorganosilicon compounds having no hydrogen bonded to silicon atoms.

Alexander et al., U.S. Pat. No. 2,892,797, describe silica sols modifiedby treatment with a solution of a metalate so that the silica particlesare coated with no more than a molecular layer of a combined metal whichforms an insoluble silicate at a pH between 5 and 12. Aluminum, tin,zinc, and lead are taught as the preferred metals. Alexander et al.teach that silica sols which carry a metal upon the surface of theparticles according to their invention have increased stability at pHextremes.

Termin et al., U.S. Pat. Nos. 3,850,971, and 4,006,175 teach that poroussilicic acid having a specific surface area of about 50 m² /g to 1000 m²/g can be made by hydrolyzing methyl or ethyl silicate or polymethyl orpolyethyl silicate with about 70 to 120% of the stoichiometric amount ofwater with moderate stirring. Termin et al. teach that transition metalssuch as iron oxides and chromium oxides may be used as hydrolysisactivators and that such metals may appear in the end product.

Nauroth et al., U.S. Pat. No. 4,360,388, teach cerium containingprecipitated silica. Nauroth et al. teach that silicone rubbercompositions reinforced with the cerium containing precipitated silicaexhibit excellent heat stability and that the cerium containingprecipitated silica acts as a fire retardant agent.

Jensen et al., EP 0-690-023 A2, teach the aging of silica gels at a pHof 4 to 11 prior to a hydrophobing step. The described silica gels arenot organosilicate modified.

Jensen et al., EP 0-658,531 A1, teach the aging of silica gels at a pHof 6 to 11 prior to a hydrophobing step. The described silica gels arenot organosilicate modified.

The hydrophobic organosilicate-modified silica gels prepared by thepresent method have improved hydrophobicity when compared to hydrophobicsilica gels prepared without the presence of the organosilicate. Theimproved hydrophobicity can make the hydrophobic organosilicate-modifiedsilica gels more compatible with organic rubber and silicone rubbercompositions. The hydrophobic organosilicate-modified silica gels alsohave a lower refractive index, which makes them desirable for use insilicone rubber compositions requiring optical clarity.

SUMMARY OF INVENTION

The present invention is neutral-aged hydrophobicorganosilicate-modified silica gels and a method for their preparation.The method comprises three steps, where in the first step anorganosilicate-modified silica hydrosol is contacted with a strongmineral acid at a pH less than about 1 to form anorganosilicate-modified silica hydrogel. In the second step theorganosilicate-modified silica hydrogel is aged at a pH within a rangeof about pH 3.5 to pH 8. In the third step the neutral-agedorganosilicate-modified silica hydrogel is contacted with anorganosilicon compound in the presence of a catalytic amount of a strongacid to effect hydrophobing of the organosilicate-modified silicahydrogel to form a hydrophobic organosilicate-modified silica hydrogelhaving a surface area within a range of about 100 m² /g to 750 m² /g inthe dry state. In a preferred process the hydrophobicorganosilicate-modified silica hydrogel is contacted with a sufficientquantity of a water-immiscible organic solvent to convert thehydrophobic organosilicate-modified silica hydrogel into a hydrophobicorganosilicate-modified silica organogel. The organic solvent can beremoved from the hydrophobic organosilicate-modified silica organogel toform a hydrophobic organosilicate-modified silica gel having a surfacearea within a range of about 100 m² /g to 750 m² /g in the dry state.

DESCRIPTION OF INVENTION

The present invention is neutral-aged hydrophobicorganosilicate-modified silica gels and a method for their preparation.The method for preparing the neutral-aged hydrophobicorganosilicate-modified silica gels comprises:

(A) contacting an organosilicate-modified silica hydrosol comprising (i)about 2 to 50 weight percent of SiO₂ per milliliter and (ii) about 1 to50 weight percent of an organosilicate described by formula R¹SiO_(3/2), where R¹ is a monovalent hydrocarbon radical comprising about1 to 6 carbon atoms, with a strong mineral acid at a pH less than about1 and at a temperature within a range of about 20° C. to 250° C. to forman organosilicate-modified silica hydrogel,

(B) aging the organosilicate-modified silica hydrogel at a pH within arange of about pH 3.5 to pH 8, and

(C) mixing the organosilicate-modified silica hydrogel with (1) acatalytic amount of a strong acid and (2) an organosilicon compoundselected from the group consisting of organosilanes described by formula

    R.sup.2.sub.a H.sub.b SiX.sub.4-a-b                        ( 1)

and organosiloxanes described by formula

    R.sup.2.sub.n SiO.sub.(4-n)/2,                             (2)

where each R² is independently selected from a group consisting ofhydrocarbon radicals comprising about 1 to 12 carbon atoms andorganofunctional hydrocarbon radicals comprising about 1 to 12 carbonatoms, each X is independently selected from a group consisting ofhalogen and alkoxy radicals comprising about 1 to 12 carbon atoms, a=0,1, 2, or 3, b=0 or 1, a+b=1, 2, or 3 with the proviso that when b=1 thena+b=2 or 3, and n is an integer of from 2 to 3 inclusive to form ahydrophobic organosilicate-modified silica hydrogel having a surfacearea within a range of about 100 m² /g to 750 m² /g as measured in thedry state.

The method of the present invention is a three-step procedure,comprising steps (A), (B), and (C), for making hydrophobicorganosilicate-modified silica gels. Step (A) of the method comprisesheating an organosilicate-modified silica hydrosol under strong acidconditions to form an organosilicate-modified silica hydrogel. Step (B)comprises aging the organosilicate-modified silica hydrogel prepared instep (A) at a pH within a range of about pH 3.5 to pH 8. Step (C)comprises mixing the neutral-aged organosilicate-modified silicahydrogel prepared in step (B) with an organosilicon compound whichreacts with the organosilicate-modified silica hydrogel to give ahydrophobic organosilicate-modified silica hydrogel. In a preferredmethod the hydrophobic organosilicate-modified silica hydrogel isfurther contacted with a sufficient amount of water-immiscible organicsolvent to convert the hydrophobic organosilicate-modified silicahydrogel to a hydrophobic organosilicate-modified silica organogel. Thewater-immiscible organic solvent can them be removed from thehydrophobic organosilicate-modified silica organogel to form ahydrophobic organosilicate-modified gel. Hydrophobicorganosilicate-modified silica gels prepared by the present method areuseful as reinforcing fillers in, for example, silicone rubber andorganic rubber compositions.

The method used to prepare the organosilicate-modified silica hydrosolis not critical and can be any of those known in the art. Theorganosilicate-modified silica hydrosol may be prepared, for example, byfirst preparing a silica hydrosol and then mixing the silica hydrosolwith an organosilicate as described herein and acidifying the mixture.The organosilicate-modified silica hydrosol may be prepared, forexample, by mixing a silica hydrosol with an acidified organosilicate.The organosilicate-modified silica hydrosol may be prepared by forming amixture of sodium silicate and the organosilicate and acidifying themixture. Silica hydrosols useful for preparing theorganosilicate-modified silica hydrosol can be prepared by, for example,deionizing sodium silicate by a method such as the use of an ionexchange resin. The silica hydrosol may be prepared by hydrolyzing asilane at a low temperature. The silica hydrosol may be prepared byacidifying a sodium silicate mixture.

Organosilicates useful in the present method are described by formula R¹SiO_(3/2), where R¹ is a monovalent hydrocarbon radical comprising about1 to 6 carbon atoms. R¹ can be, for example, alkyls such as methyl,ethyl, and hexyl; substituted alkyls such as 3,3,3-trifluoropropyl andchloromethyl; alkenyls such as vinyl, allyl, and hexenyl; and aryls suchas phenyl.

The organosilicate-modified silica hydrosol requires the presence ofabout 2 to 50 weight percent of SiO₂ per milliliter (ml) of thehydrosol. Preferred is when the organosilicate-modified silica hydrosolcomprises about 5 to 20 weight percent of SiO₂ per ml of the hydrosol.The organosilicate-modified silica hydrosol also requires the presenceof about 1 to 50 weight percent of an organosilicate described byformula R¹ SiO_(3/2), where R¹ is as described above, per ml of thehydrosol. Preferred is when the organosilicate-modified silica hydrosolcomprises about 1 to 20 weight percent of the organosilicate per ml ofthe hydrosol.

In step (A), the organosilicate-modified silica hydrosol must becontacted with a sufficient concentration of a strong mineral acid suchthat the pH of the organosilicate-modified silica hydrosol is less thanabout pH 1. Preferably, there should be a sufficient amount of thestrong mineral acid present so that the pH is essentially 0, that is sothat the pH cannot be measured. For the purpose of this invention anystrong mineral acid can be used. As used herein, the term "strongmineral acid" refers to those acids which ionize to the extent of atleast 25 percent in 0.1N aqueous solution at 18° C. The strong mineralacid may be, for example, hydrochloric, hydroiodic, sulfuric, nitric,and phosphoric acid.

In step (A), the organosilicate-modified silica hydrosol is contactedwith the strong mineral acid at a temperature within a range of about20° C. to 250° C. Preferred is when the organosilicate-modified silicahydrosol is contacted with the strong mineral acid at a temperaturewithin a range of about 20° C. to 80° C. Even more preferred is when, instep (A), the organosilicate-modified silica hydrosol is contacted withthe strong mineral acid at a temperature within a range of about 20° C.to 50° C.

In step (A), the contacting time required varies with the temperatureand acid concentration. Generally the higher the temperature and thegreater the acid concentration the shorter the contact time needed. Thecontacting of step (A) must be continued until theorganosilicate-modified silica hydrogel acquires a structure such thatthe final product after hydrophobing has a surface area in the dry statewithin a range of about 100 m² /g to 750 m² /g as determined by theBrunauer Emmett and Teller (BET) method described in the Jour. Am. Chem.Soc. 60:309 (1938) and as further described in Lentz, U.S. Pat. No.3,122,520, which is hereby incorporated by reference for such ateaching. The surface area of the organosilicate-modified silicahydrogel at the conclusion of step (A) is immaterial provided it is suchthat the surface area of the dried product after the hydrophobing ofstep (C) is within the above described range. Generally the surface areaof the organosilicate-modified silica hydrogel is reduced by thehydrophobing reaction, since the organosilyl groups which becomeattached to the surface of the organosilicate-modified silica hydrogelincrease the average particle size. The surface of theorganosilicate-modified silica hydrogel can be above 750 m² /g providedthat the hydrophobing treatment brings it within a range of about 100 m²/g to 750 m² /g.

To determine the proper contact conditions during conduct of step (A) itis necessary to proceed with step (B) and the hydrophobing of step (C)and then measure the surface area of the resulting product in the drystate. If the surface area of the resulting product in the dry state isabove 750 m² /g, then the contacting conditions of step (A) were toomild. If the surface area of the resulting product in the dry state isbelow 100 m² /g, then the contacting conditions of step (A) were toosevere. Examples of suitable acid concentrations, temperatures, andtimes for conduct of step (A) are provided in the Examples herein. Ifthe surface area of the hydrophobic organosilicate-modified silica gelin the dry state is above or below the described range, the hydrophobicorganosilicate-modified silica gels have diminished reinforcingproperties in silicone elastomers.

In step (B) of the present method, the organosilicate-modified silicahydrogel of step (A) is aged at a pH within a range of about pH 3.5 topH 8. Preferred is when the organosilicate-modified silica hydrogel isaged at a pH within a range of about pH 6 to pH 7.5. The pH of theorganosilicate-modified silica hydrogel can be adjusted to within thedescribed ranges by use of a base such as NH₄ OH, NaOH, KOH, and Na₂O(SiO₂)₃.36. Preferred is when the organosilicate-modified silicahydrogel of step (A) is first washed with deionized water to removeelectrolytes provided by the strong mineral acid, then the pH isadjusted to within the described ranges. Generally, theorganosilicate-modified silica hydrogel can be aged at a temperaturewithin a range of about 0° C. to 250° C. It is preferred that theorganosilicate-modified silica hydrogel be aged at a temperature withina range of about 20° C. to 150° C. Most preferred is when theorganosilicate-modified silica hydrogel is aged at a temperature withina range of about 20° C. to 80° C. The length of time for aging theorganosilicate-modified silica hydrogel can be from about 10 minutes to76 hours or longer. A preferred length of time for aging theorganosilicate-modified silica hydrogel is within a range of about 1hour to 24 hours.

If desired, the organosilicate-modified silica hydrogel of step (B) maybe subjected to a shearing force to reduce aggregate particle size andcreate a more uniform particle size distribution prior to the conduct ofthe hydrophobing reaction of step (C). The shearing force may be appliedto the organosilicate-modified silica hydrogel by any of those methodsknown in the art. The shearing force may be applied, for example, by amechanical means such as a high-speed mixer or by ultrasound. Thisreduction in aggregate particle size and improved uniformity of particlesize can provide for hydrophobic organosilicate-modified silica gelswhich when compounded into silicone elastomer compositions provide forlower viscosity compositions, more stable compositions, and for curedsilicone elastomers having improved clarity and physical properties.

In step (C) of the present method the neutral-agedorganosilicate-modified silica hydrogel of step (B) is mixed with one ormore of the defined organosilicon compounds described by formulas (1)and (2) in the presence of a catalytic amount of a strong acid. In step(C), the strong acid can be the same acid which was used in step (A).The catalytic amount of strong acid can be added either prior to,simultaneously with, or subsequent to the addition of the organosiliconcompound. In the case where the organosilicon compound is, for example,a chlorosilane, the catalytic amount of the strong acid can be generatedin situ by hydrolysis of the chlorosilane or the reaction of thechlorosilane directly with hydroxyls of the organosilicate-modifiedsilica hydrogel. In step (C) the limitations on pH as described for step(A) do not apply. It is only necessary that a catalytic amount of astrong acid be present in an amount sufficient to effect reaction of theorganosilicon compound with the organosilicate-modified silica hydrogel.Examples of useful acids include hydrochloric, sulfuric, and benzenesulfonic acids. It is preferred that in step (C) the strong acidcatalyst provide a pH less than about 2.5.

The temperature at which the hydrophobing of step (C) is conducted isnot critical and can be from about 20° C. to 250° C. Generally it ispreferred that the hydrophobing of step (C) be conducted at atemperature within a range of about 30° C. to 150° C. The hydrophobingof step (C) can be conducted at the reflux temperature of thewater-immiscible organic solvent when it is present.

In step (C), the organosilicate-modified silica hydrogel of step (B) isreacted with an organosilicon compound described by formulas (1) and(2). In formulas (1) and (2), each R² can be independently selected froma group consisting of hydrocarbon radicals comprising about 1 to 12carbon atoms and organofunctional hydrocarbon radicals comprising about1 to 12 carbon atoms. R² can be a saturated or unsaturated hydrocarbonradical. R² can be a substituted or non-substituted hydrocarbon radical.R² can be, for example, alkyl radicals such as methyl, ethyl, t-butyl,hexyl, heptyl, octyl, decyl, and dodecyl; alkenyl radicals such asvinyl, allyl, and hexenyl; substituted alkyl radicals such aschloromethyl, 3,3,3-trifluoropropyl, and 6-chlorohexyl; and arylradicals such as phenyl, naphthyl, and tolyl. R² can be anorganofunctional hydrocarbon radical comprising about 1 to 12 carbonatoms where, for example, the functionality is mercapto, disulfide,polysulfide, amino, carboxylic acid, carbinol, ester, or amido. Apreferred organofunctional hydrocarbon radical is one having disulfideor polysulfide functionality.

In formula (1) each X is independently selected from a group consistingof halogen and alkoxy radicals comprising about 1 to 12 carbon atoms.When X is a halogen, it is preferred that the halogen be chlorine. WhenX is an alkoxy radical, X may be, for example, methoxy, ethoxy, andpropoxy. Preferred is where each X is selected from a group consistingof chlorine atoms and methoxy.

The viscosity of the organosiloxanes described by formula (2) is notlimiting and can range from that of a fluid to a gum. Generally, highermolecular weight organosiloxanes will be cleaved by the acidicconditions of the present method allowing them to react with theorganosilicate-modified silica hydrogel.

The organosilicon compound may be provided to the present method as asingle compound as described by formula (1) or (2) or as a mixture oftwo or more organosilicon compounds described by formulas (1) and (2).

Examples of useful organosilicon compounds includediethyldichlorosilane, allylmethyldichlorosilane,methylphenyldichlorosilane, phenylethyldiethoxysilane,3,3,3-trifluoropropylmethyldichlorosilane, trimethylbutoxysilane,symdiphenyltetramethyldisiloxane, trivinyltrimethylcyclotrisiloxane,hexaethyldisiloxane, pentylmethyldichlorosilane, divinyldipropoxysilane,vinyldimethylchlorosilane, vinylmethyldichlorosilane,vinyldimethylmethoxysilane, trimethylchlorosilane, hexamethyldisiloxane,hexenylmethyldichlorosilane, hexenyldimethylchlorosilane,dimethylchlorosilane, dimethyldichorosilane,mercaptopropylmethyldimethoxysilane, andbis{3-(triethoxysilyl)propyl}tetrasulfide. When the hydrophobicorganosilicate-modified silica gel is to be used as a filler in siliconerubber, it is preferred that the organosilicon compound behexamethyldisiloxane or dimethyldichlorosilane.

The amount of organosilicon compound added to the method is thatsufficient to adequately hydrophobe the organosilicate-modified silicahydrogel to provide a hydrophobic organosilicate-modified silica gelsuitable for its intended use. Generally the organosilicon compoundshould be added to the method in an amount such that there is at least0.04 organosilyl unit per SiO₂ unit in the organosilicate-modifiedsilica hydrogel. The upper limit of the amount of organosilicon compoundadded to the process is not critical since any amount in excess of theamount required to saturate the organosilicate-modified silica gel willact as a solvent for the method.

The hydrophobic organosilicate-modified silica hydrogel of step (C) maybe used as is or may be recovered for use by such methods ascentrifugation or filtration. The hydrophobic organosilicate-modifiedsilica hydrogel may be dried by the use of such methods as heating orreducing pressure or a combination of both heating and reduced pressure.

In a preferred method a water-immiscible organic solvent in sufficientamount to convert the organosilicate-modified silica hydrogel orhydrophobic organosilicate-modified silica hydrogel to the correspondingorganogel is added to the method. The solvent can be added prior to,simultaneously with, or subsequent to the addition of the organosiliconcompound. That is, the organosilicate-modified silica hydrogel can befirst converted into an organogel by replacement of the water with theorganic solvent and then hydrophobed. On the other hand, theorganosilicon compound and the organic solvent can be addedsimultaneously to the organosilicate-modified silica hydrogel. Underthese conditions the reaction of the organosilicate-modified silicahydrogel with the organosilicon compound and the replacement of thewater in the hydrophobic organosilicate-modified silica hydrogel withthe organic solvent may occur simultaneously. Finally the organosiliconcompound can be added prior to the organic solvent, in which case theorganosilicate-modified silica hydrogel reacts with the organosiliconcompound and the resulting product is then converted into an organogelby an addition of an organic solvent. In the latter two cases, theconversion to an organogel is accomplished by a phase separation inwhich the hydrophobic organosilicate-modified silica gel passes into thewater-immiscible organic solvent phase. A preferred method is where awater-immiscible organic solvent is added after the formation of thehydrophobic organosilicate-modified silica hydrogel thereby effectingformation of a hydrophobic organosilicate-modified silica organogel.

For purpose of this invention any organic solvent immiscible with watercan be employed. Suitable solvents include low molecular weightsiloxanes such as hexamethyldisiloxane, octamethylcyclotetrasiloxane,diphenyltetramethyldisiloxane and trimethylsilyl endblockeddimethylpolysiloxane fluids. When a siloxane is employed as a solvent itmay serve both as a solvent and as a reactant with theorganosilicate-modified silica hydrogel. In addition, suitable solventsinclude aromatic hydrocarbons such as toluene and xylene; heptane andother aliphatic hydrocarbon solvents; cycloalkanes such as cyclohexane;ethers such as diethylether and dibutylether; halohydrocarbon solventssuch as methylene chloride, chloroform, ethylene chloride, andchlorobenzene; and ketones such as methylisobutylketone.

The amount of water-immiscible organic solvent is not critical so longas there is sufficient solvent to convert the hydrophobicorganosilicate-modified silica hydrogel into a silica organogel.Preferably the solvent should have a boiling point below about 250° C.to facilitate its removal from the hydrophobic organosilicate-modifiedsilica organogel, however the boiling point is not critical since thesolvent may be removed from the hydrophobic organosilicate-modifiedsilica organogel by centrifuging, evaporation, or other suitable means.

After the organosilicate-modified silica hydrogel has been converted tothe hydrophobic organosilicate-modified silica organogel the resultingproduct may be employed per se. That is the hydrophobicorganosilicate-modified silica organogel may be used directly as areinforcing agent in silicone rubber or in any other uses for which thistype of product can be used. Alternatively, the solvent may be removedfrom the hydrophobic organosilicate-modified silica organogel and theresulting dry hydrophobic organosilicate-modified silica gel used.

During the conduct of step (C) it may by desirable to add a surfactantor water immiscible solvent to facilitate the reaction of theorganosilicon compound with the organosilicate-modified silica hydrogel.The surfactant or water-miscible solvent may be added in the presence orabsence of any water-immiscible organic solvent added to the method.Suitable surfactants may include, for example, anionic surfactants suchas dodecylbenzene sulfonic acid, nonionic surfactants such aspolyoxyethylene(23)lauryl ether and (Me₃ SiO)₂ MeSi(CH₂)₃ (OCH₂ CH₂)₇OMe where Me is methyl, and cationic surfactants such asN-alkyltrimethyl ammonium chloride. Suitable water-miscible solventsinclude, for example, alcohols such as ethanol, propanol, isopropanol,and tetrahydrofuran.

In step (C) of the present method an effective amount of a heatstabilizing agent selected from a group consisting of water solublecompounds of cerium and iron may be added. By the term "effectiveamount" it is meant that the water soluble compound of cerium or iron ispresent in the hydrophobic organosilicate-modified silica gel at aconcentration sufficient to provide improved heat stability to thosecompositions in which the hydrophobic organosilicate-modified silica gelis incorporated. Such compositions can include, for example, siliconerubber, natural rubber, and synthetic organic rubber.

Generally, about 0.01 percent weight/volume (% Wt./Vol.) to 10% Wt./Vol.of the water soluble compound of cerium or iron in relation to thevolume of components in step (C), excluding solvents, is considereduseful in the present process. Preferred is where the water solublecompound of cerium or iron comprises about 0.1% Wt./Vol. to 1% Wt./Vol.on the same basis.

Examples of water soluble compounds which may be useful in the presentmethod include FeCl₃, FeBr₂, FeBr₃.6H₂ O, FeCl₂.4H₂ O, FeI₂.4H₂ O,Fe(NO₃)₃.6H₂ O, FePO₄.2H₂ O, CeCl₃.9H₂ O, CeBr₃.H₂ O, CeI₃.9H₂ O,Ce(NO₃)₃.6H₂ O, and Ce(SO₄)₂.2H₂ O. A preferred water soluble compoundof cerium and iron for use in the present method is selected from thegroup consisting of FeCl₃ and CeCl₃.9H₂ O.

The following examples are provided to illustrate the present invention.These examples are not intended to limit the scope of the presentclaims.

EXAMPLE 1

An organosilicate-modified silica gel aged at pH 6.8, hydrophobed withhexamethyldisiloxane, and having incorporated therein FeCl₃ as a heatstabilizing agent was prepared. A solution was prepared comprising 312ml of PQ N Clear Sodium Silicate (PQ Corporation, Valley Forge, Pa.),140 ml of sodium methyl silicate (DC® 722, Dow Corning Corporation,Midland, Mich.), and 948 ml of deionized water. This solution was addedto a rapidly stirred solution comprising 300 ml of concentrated HCl(Fisher Certified, Fisher Scientific, Fair Lawn, N.J.) diluted with 300ml of deionized water to form an organosilicate modified silica hydrosolcomprising 0.063 g of SiO₂ /ml and 0.017 g of MeSiO_(3/2) /ml (Merepresents a methyl group). After stirring for an additional 2 to 3minutes, the organosilicate-modified silica hydrosol was poured intoflat pans and allowed to gel for about 2.5 hours. Theorganosilicate-modified silica hydrogel was cut into 2.5 cm squares andwashed with deionized water until the pH of the effluent was between pH3 and pH 4. The washed organosilicate-modified silica hydrogel wasplaced in a glass container, adjusted to about pH 6.8 by the addition ofconcentrated ammonium hydroxide, and the resulting mixture aged 44 hoursat room temperature.

After neutral aging, the water phase was drained from theorganosilicate-modified silica gel and the gel placed in a 5 L glassflask. To the organosilicate-modified silica hydrogel, with stirring,was added 727 ml of concentrated HCl (Fisher Certified), 909 ml ofisopropanol, 471 ml of hexamethyldisiloxane, and 4.2 g of FeCl₃. Afterstirring the flask content 1 hour at room temperature, 2 L of toluenewere added. After stirring the flask content for an additional 2 to 3minutes, stirring was stopped and the aqueous phase drained from theflask. The toluene phase was washed with 1 L of deionized water. Theflask was fitted with a Dean-Stark trap and the toluene phase refluxedto remove residual water. The toluene phase was evaporated under reducedpressure leaving as product a hydrophobic organosilicate-modified silicagel. The organosilicate-modified hydrophobic silica gel was dried for 14hours at 150° C. The yield of dried hydrophobic organosilicate-modifiedsilica gel was 171 g. The BET surface area of the dried hydrophobicorganosilicate-modified silica gel was determined by the methoddescribed supra; void volume, pore volume, average pore diameter, andparticle size were characterized by standard methods; and carbon contentwas determined by CHN analysis using a Perkin Elmer Model 2400 CHNElemental Analyzer (Perkin Elmer Corporation, Norwalk, Conn.). Theresult of these analysis are reported in Table 1.

EXAMPLE 2

An organosilicate-modified silica gel aged at pH 6.8, hydrophobed withdimethyldichlorosilane, and having incorporated therein FeCl₃ as a heatstabilizing agent was prepared. A solution was prepared comprising 312ml of PQ N Clear Sodium Silicate (PQ Corporation), 140 ml of sodiummethyl silicate (DC 722) and 948 ml of deionized water. This solutionwas added to a rapidly stirred solution comprising 300 ml ofconcentrated HCl (Fisher Certified) diluted with 300 ml of deionizedwater to form an organosilicate-modified silica hydrosol comprising0.063 g of SiO₂ /ml and 0.017 g MeSiO_(3/2) /ml (Me represents a methylgroup). After stirring for an additional 2 to 3 minutes, theorganosilicate-modified silica hydrosol was poured into flat pans andallowed to gel for about 2 hours. The organosilicate-modified silicahydrogel was cut into 2.5 cm squares and washed with deionized wateruntil the pH of the effluent was between pH 3 and pH 4. The washedorganosilicate-modified silica hydrogel was placed in a glass container,adjusted to about pH 6.8 by the addition of concentrated ammoniumhydroxide, and the resulting mixture aged 44 hours at room temperature.

After neutral aging, the water phase was drained from theorganosilicate-modified silica gel and the gel placed in a 5 L glassflask. To the organosilicate-modified silica hydrogel, with stirring,was added 727 ml of concentrated HCl (Fisher Certified), 909 ml ofisopropanol, 130 ml of dimethyldichlorosilane, and 4.2 g of FeCl₃. Afterstirring the flask content 1 hour at room temperature, 2 L of toluenewere added to the flask. After stirring the flask content for anadditional 2 to 3 minutes, stirring was stopped and the aqueous phasedrained from the flask. The toluene phase was washed with 1 L ofdeionized water. The flask was fitted with a Dean-Stark trap and thetoluene phase refluxed to remove residual water. The toluene phase wasevaporated under reduced pressure leaving as product a hydrophobicorganosilicate-modified silica gel. The organosilicate-modifiedhydrophobic silica gel was dried 14 hours at 150° C. The yield of driedhydrophobic organosilicate-modified silica gel was 179 g. Physicalproperties of the dried hydrophobic organosilicate-modified silica gelwere determined by the methods described in Example 1 and the resultsare reported in Table 1.

EXAMPLE 3

An organosilicate-modified silica gel aged at pH 6.8 and hydrophobedwith hexamethyldisiloxane was prepared. The neutral-agedorganosilicate-modified silica gel was sheared prior to hydrophobing toreduce aggregate particle size and improve the uniformity of theparticle size distribution. A solution was prepared comprising 156 ml ofPQ N Clear Sodium silicate (PQ Corporation), 70 ml of sodium methylsilicate (DC 722), and 474 ml of deionized water. This solution wasadded to a rapidly stirred solution comprising 150 ml of concentratedHCl (Fisher Certified) diluted with 150 ml of deionized water to form anorganosilicate-modified silica hydrosol comprising 0.063 g of SiO₂ /mland 0.017 g of MeSiO_(3/2) /ml (Me represents a methyl group). Afterstirring for an additional 2 to 3 minutes, the organosilicate-modifiedsilica hydrosol was poured into flat pans and allowed to gel for about 2hours. The resulting organosilicate-modified silica hydrogel was cutinto 2.5 cm squares and washed with deionized water until the pH of theeffluent was about pH 2.2. The washed organosilicate-modified silicahydrogel was placed in a glass container, adjusted to about pH 6.8 bythe addition of concentrated ammonium hydroxide, and the resultingmixture aged 44 hours at room temperature.

After neutral aging the organosilicate-modified silica hydrogel thewater phase was drained-off and 364 ml of concentrated HCl (FisherCertified) added. The acidified organosilicate-modified silica hydrogelwas placed in a Waring Blender (Model 7011, Waring Products Division ofDynamics Corporation of America, New Hartford, Conn.) and theorganosilicate-modified silica hydrogel sheared for two minutes. Thesheared organosilicate-modified silica hydrogel was then placed in a 5 Lglass flask and, with stirring, was added 909 ml of isopropanol and 115ml of hexamethyldisiloxane. After stirring the flask content for 1 hourat room temperature, 1.3 L of toluene were added to the flask. Afterstirring the flask content for an additional 2 to 3 minutes, stirringwas stopped and the aqueous phase drained from the flask. The toluenephase was washed with 0.5 L of deionized water. The flask was fittedwith a Dean-Stark trap and the toluene phase refluxed to remove residualwater. The toluene phase was evaporated under reduced pressure leavingas product a hydrophobic organosilicate-modified silica gel. Theorganosilicate-modified hydrophobic silica gel was dried for 14 hours at150° C. The yield of dried hydrophobic organosilicate-modified silicagel was 77 g. Physical properties of the dried organosilicate-modifiedsilica gel was determined by the methods described in Example 1 and theresults are reported in Table 1.

EXAMPLE 4

An organosilicate-modified silica gel aged at pH 6.8 and hydrophobedwith dimethyldichlorosilane was prepared. The neutral-agedorganosilicate-modified silica gel was sheared prior to hydrophobing toreduce aggregate particle size and improve the uniformity of theparticle size distribution. A solution was prepared comprising 156 ml ofPQ N Clear Sodium silicate (PQ Corporation), 70 ml of sodium methylsilicate (DC 722), and 474 ml of deionized water. This solution wasadded to a rapidly stirred solution comprising 150 ml of concentratedHCl (Fisher Certified) diluted with 150 ml of deionized water to form anorganosilicate-modified silica hydrosol comprising 0.063 g of SiO₂ /mland 0.017 g of MeSiO_(3/2) /ml (Me represents a methyl group). Afterstirring for an additional 2 to 3 minutes the organosilicate-modifiedsilica hydrosol was poured into flat pans and allowed to gel for about 2hours. The organosilicate-modified silica hydrogel was cut into 2.5 cmsquares and washed with deionized water until the pH of the effluent wasabout pH 2.2. The washed organosilicate-modified silica hydrogel wasplaced in a glass container, adjusted to about pH 6.8 by the addition ofconcentrated ammonium hydroxide, and the resulting mixture aged 44 hoursat room temperature.

After neutral aging, the water phase was drained from theorganosilicate-modified silica hydrogel and 364 ml of concentrated HCl(Fisher Certified) added. The acidified organosilicate-modified silicahydrogel was placed in a Waring Blender (Model 7011) and sheared for twominutes. The sheared organosilicate-modified silica hydrogel was thenplaced in a 5 L glass flask and, with stirring, was added 455 ml ofisopropanol and 69 ml of dimethyldichlorosilane. After stirring theflask content for 1 hour at room temperature, 1.3 L of toluene wereadded to the flask. After stirring the flask content for an additional 2to 3 minutes, stirring was stopped and the aqueous phase drained fromthe flask. The toluene phase was washed with 0.5 L of deionized water.The flask was fitted with a Dean-Stark trap and the toluene phaserefluxed to remove residual water. The toluene phase was evaporatedunder reduced pressure leaving as product a hydrophobicorganosilicate-modified silica gel. The organosilicate-modifiedhydrophobic silica gel was dried for 14 hours at 150° C. The yield ofdried hydrophobic organosilicate-modified silica gel was 92 g. Physicalproperties of the dried organosilicate-modified silica gel weredetermined by the methods described in Example 1 and the results arereported in Table 1.

EXAMPLE 5

The dried hydrophobic organosilicate-modified silica gel prepared inExample 1 was compounded into a curable silicone rubber composition, thecomposition cured, and the physical properties determined. A driedhydrophobic organosilicate-modified silica gel prepared as described inExample 1 was compounded at 38 parts per hundred (pph) into apolydimethylsiloxane gum containing about 0.15 mole percent vinylradicals substituted on silicon atoms and having a plasticity of about55 to 65. Into this base composition was blended 0.7 pph of2,5-bis(tert-butylperoxyl)-2,5-dimethylhexane, based on the weight ofthe polydimethylsiloxane gum. The catalyzed base composition was curedin appropriate configurations for physical property testing by hotpressing at 34.5 MPa for 15 minutes at 175° C. The following testmethods were used to test the cured silicone rubber: Tensile ASTM D412;Elongation, ASTM D412; 50% Modulus, ASTM D412; 100% Modulus, ASTM D412;Durometer (Shore A), ASTM 2240; Tear (Die B), ASTM D624; Tear (Die C),ASTM D624; and Compression set (22 h at 177° C.), ASTM D395. The resultsof this testing are reported in Table 1.

                  TABLE 1                                                         ______________________________________                                        Properties of Neutral-Aged Organosilicate-Modified                            Silica Gels and Elastomers Prepared Therefrom                                            Example Number                                                     Property     1        2        3      4                                       ______________________________________                                        BET Surface Area (m.sup.2 /g)                                                              701      428      631    440                                     Void Volume (cm.sup.3 /g)                                                                  7.15     5.82     7.73   7.27                                    Pore Volume (cm.sup.3 /g)                                                                  3.21     2.93     3.11   3.07                                    Ave. Pore Diameter (Å)                                                                 129      185      144    188                                     Particle Size (μm)                                                                      47.5     84.2     42.3   68.1                                    Wt. % Carbon 14.0     12.2     10.7   12.5                                    Tensile (MPa)                                                                              8.54      --*     --     --                                      Elongation (%)                                                                             846      --       --     --                                      Modulus @50% (MPa)                                                                         0.48     --       --     --                                      Modulus @100% (MPa)                                                                        0.66     --       --     --                                      Durometer (Shore A)                                                                        46       --       --     --                                      Tear Die B (kN/m)                                                                          12.20    --       --     --                                      Tear Die C (kN/m)                                                                          11.66    --       --     --                                      Compression Set (%)                                                                        22.6     --       --     --                                      ______________________________________                                         *Indicates data not available                                            

We claim:
 1. A method for preparing a neutral-aged hydrophobicorganosilicate-modified silica gel comprising:(A) contacting anorganosilicate-modified silica hydrosol comprising (i) about 2 to 50weight percent of SiO₂ per milliliter and (ii) 1 to 50 weight percent ofan organosilicate described by formula R¹ SiO_(3/2), where R¹ is amonovalent hydrocarbon radical comprising about 1 to 6 carbon atoms,with a strong mineral acid at a pH less than about 1 and at atemperature within a range of about 20° C. to 250° C. to form anorganosilicate-modified silica hydrogel, (B) aging theorganosilicate-modified silica hydrogel at a pH within a range of aboutpH 3.5 to pH 8 for about 10 minutes to 76 hours, and (C) mixing theorganosilicate-modified silica hydrogel with (1) a catalytic amount of astrong acid and (2) an organosilicon compound selected from the groupconsisting of organosilanes described by formula

    R.sup.2.sub.a H.sub.b SiX.sub.4-a-b

and organosiloxanes described by formula

    R.sup.2.sub.n SiO.sub.(4-n)/2,

where each R² is independently selected from a group consisting ofhydrocarbon radicals comprising about 1 to 12 carbon atoms andorganofunctional hydrocarbon radicals comprising about 1 to 12 carbonatoms, each X is independently selected from a group consisting ofhalogen and alkoxy radicals comprising 1 to 12 carbon atoms, a=0, 1, 2,or 3, b=0 or 1, a+b=1, 2, or 3 with the proviso that when b=1 then a+b=2or 3, n is an integer of from 2 to 3 inclusive to form a hydrophobicorganosilicate-modified silica hydrogel having a surface area within arange of about 100 m² /g to 750 m² /g as measured in the dry state.
 2. Amethod according to claim 1, where R¹ is methyl.
 3. A method accordingto claim 1, where the organosilicate-modified silica hydrosol comprisesabout 5 to 20 weight percent of SiO₂ per ml of the hydrosol and about 1to 20 weight percent of the organosilicate per ml of the hydrosol.
 4. Amethod according to claim 1, where in step (A) theorganosilicate-modified silica hydrosol is contacted with the strongmineral acid at a temperature within a range of about 20° C. to 80° C.5. A method according to claim 1, where in step (A) theorganosilicate-modified silica hydrosol is contacted with the strongmineral acid at a temperature within a range of about 20° C. to 50° C.6. A method according to claim 1, where the aging of step (B) isconducted at a temperature within a range of about 0° C. to 250° C. fora length of time of from about 10 minutes to 76 hours.
 7. A methodaccording to claim 1, where the aging of step (B) is conducted at a pHwithin a range of about pH 6 to pH 7.5 and a temperature within a rangeof about 20° C. to 150° C. for a length of time within a range of about1 hour to 24 hours.
 8. A method according to claim 1 further comprisingshearing the organosilicate-modified silica hydrogel of step (B) priorto conduct of step (C).
 9. A method according to claim 1, where in step(C) the catalytic amount of strong acid provides a pH less than about2.5.
 10. A method according to claim 1, where the mixing of step (C) isconducted at a temperature within a range of about 30° C. to 150° C. 11.A method according to claim 1, where the organosilicon compound is anorganosiloxane.
 12. A method according to claim 11, were theorganosiloxane is hexamethyldisiloxane.
 13. A method according to claim1, where the organosilicon compound is an organosilane.
 14. A methodaccording to claim 13, where the organosilane is selected from the groupconsisting of vinyldimethylchlorosilane, vinylmethyldichlorosilane,dimethyldichlorosilane, hexenylmethyldichlorosilane,hexenyldimethylchlorosilane, andbis{3-(triethoxysilyl)propyl}tetrasulfide.
 15. A method according toclaim 1, where the organosilicon compound provides at least 0.04organosilyl units per SiO₂ unit of the organosilicate-modified silicahydrogel.
 16. A method according to claim 1 further comprisingcontacting the hydrophobic organosilicate-modified silica hydrogel witha water-immiscible organic solvent in sufficient amount to convert thehydrophobic organosilicate-modified silica hydrogel to a hydrophobicorganosilicate-modified silica organogel.
 17. A method according toclaim 1 further comprising during the mixing of step (C) the presence ofa surfactant which facilitates reaction of the organosilicon compoundwith the organosilicate-modified silica hydrogel.
 18. A method accordingto claim 1 further comprising during the mixing of step (C) the presenceof a water-miscible solvent which facilitates reaction of theorganosilicon compound with the organosilicate-modified silica hydrogel.19. A method according to claim 1 further comprising mixing theorganosilicate-modified hydrogel with an effective amount of a heatstabilizing agent selected from the group consisting of water solublecompounds of cerium and iron.
 20. A method according to claim 19, wherethe water soluble compound is selected from the group consisting ofFeCl₃ and CeCl₃.9H₂ O.